What is Subnetting?

Subnetting allows you
to create multiple logical networks that exist within a single Class A, B, or C
network.

To subnet a network is
to create logical divisions of the network. Subnetting, therefore, involves
dividing the network into smaller portions called subnets. Subnetting applies
to IP addresses because this is done by borrowing bits from the host portion of
the IP address. In a sense, the IP address then has three components - the
network part, the subnet part and, finally, the host part.

For Example:

11111111.11111111.11111111.11110000
will be equal to 255.255.255.240 in decimal.

Where, 11111111 = 255

And, 11110000 = 240

In 11111111 the total
bits in network field are 8. So, 2^8=255 and,

For 11110000, Notice that the fourth byte in the network mask has four bits set
to ‘1’. If you look at the chart below, its value is 240 (128+64+32+16).

There are so many
reasons why we subnet:

a. It helps in the
preservation of address space in other not to waste addresses.

b. It used for security.

c. It helps to control network traffic due to collisions of packets transmitted
by other node (host) on the same segment.

Subnetting a Network
Address.

In order to subnet a
network address, The subnet mask has to be extended, using some of the
bits from the host ID portion of the address to create a sub network ID.

How to Subnet:

This mean, borrow some
bits from the host part to add to the network part. This allows us to have more
networks than using the default subnet mask. For example, we can borrow some
bits in the next octet to make the address belong to a different network.

For example:

Now you can clearly
see that the subnet mask will decide which is the network part, which is the
Host part. By borrowing 8 bits, our subnet mask will be like this:

After changing the
second octet of the subnet mask from all “0″ to all “1″, the Network part is
now extended. Now we can create new networks by changing number in the first or
second octet. This greatly increases the number of networks we can create.

So, in conclusion we
“subnet” by borrowing bit “0″ in the Host portion and converting them to bit
“1″. The number of borrowed bits is depended on how many networks we need.